Traditional optical metrology is intended for measurement of optics with low aberrations. Attempts to adapt traditional metrology to measurement of highly aberrated wavefronts and surfaces meet several limitations. Conventional metrology with plane or spherical reference produces fringe patterns with high spatial frequencies, which cannot be resolved by the interferometer imaging sensor. Aspheric null mirrors pose the problem of fabrication, testing the null itself, and alignment.
It would instead be desirable to use one simple shape of the reference optics for all shapes of highly aberrated optics to be tested, allow for relaxed tolerances on the reference, have low requirements to the resolution of the imaging sensor, and feature easy fabrication, testing, and alignment of all components of the metrology system and method.
The invention disclosed herein will serve as an attachment to a standard interferometer. It is planned for use by optical fabrication facilities in production of optics, both aspheric and spherical, and highly aberrated optics in general. The hardware of this invention will be compatible with standard commercial interferometers already in use by fabrication facilities. The metrology according to this invention will further use standard procedures from the operator standpoint, to reduce “psychological threshold” for its application in existing manufacturing processes. One of the distinctive features of the invention is the low cost.
The system and method according to this invention are uniquely beneficial for testing existing and emerging optics designed with high aberrations. It will find applications to new types of highly aberrated, aspheric optics. For example, as precision single-point diamond turning continues to grow popular and moldable infrared glasses enable new types of optical systems, surface shapes previously considered exotic are becoming mainstream. Compound lenses will continue to emerge with aspheric components that have intentionally high aberrations prior to assembly. While testing the assembled lens is possible with traditional interferometers, the metrology according to this invention, performed on individual components, may enable otherwise impossible tests at earlier fabrication stages, to improve quality, increase productivity, and reduce cost. The ability of this invention to measure highly aberrated optics will enable new optical designs which are presently difficult to implement and impossible to test.
Many interferometers are commercially available, applicable to measurement of wavefronts and optical figures of optical components. Common to all methods of interferometry are the following requirements: 1) the setup must get light back into the interferometer; 2) the sensor must be able to resolve the fringes; and 3) the optical test setup must be precisely defined for calculation of the wavefront.
Transparent domes introduce significant aberrations into transmitted wavefronts. With the exception of a spherical dome illuminated by a spherical wavefront concentric with the dome, wavefront aberrations are always present and significant, especially in deep concave shapes typical of aerodynamically conformal domes. For example, the corrector optics inside a missile dome is designed to compensate these aberrations. The compensation typically varies with the look angle of the gimbaled corrector optics inside the dome. Therefore, unlike production and testing of low-aberration optics, the task of fabricating and testing aspheric domes and associated corrector optics is to make and test optics “highly aberrated by design.” Traditional interferometry is not directly applicable to this task; it serves to measure small amounts of aberrations. At large aberrations, the spatial frequency of the fringe pattern exceeds the Nyquist limit, so that the pattern is undersampled by the imaging sensor. This causes either complete failure of the measurement or loss of confidence in the result, e.g., due to assumptions about the wavefront shape made in sub-Nyquist sampling, such as described by Lerner et al (Scott A. Lerner. Jose M. Sasian. John E. Greivenkamp Robert 0. Gappinger. Steve R. Clark. Interferometric Metrology of Conformal Domes. April 1999 SPIE Vol. 3705, pp. 221-226, 1999). The same paper analyzed several layouts of interferometric transmitted wavefront testing. Of the multiple options considered, identified as promising were the sub-aperture stitching and aspheric null for full-aperture testing. Testing with full-frame registration and spherical reference was rejected due to unresolved fringes.
Null mirrors may be used to back-reflect the aberrated wavefront, so that for the “proper” aberrations, the surface of the null reflector is coincident with the aberrated wavefront. The three major difficulties of this approach are fabrication, testing, and alignment. While fabrication of aspheric rotationally-symmetric nulls is possible with modern single-point diamond turning, their testing is a difficult task. The optical figure of the null reference needs to be guaranteed with accuracy higher than that of the conformal dome. Therefore, the task of testing the null is even more challenging than that of testing the dome itself. Finally, aspheric optics tends to be more sensitive to all types of misalignment, compared to conventional spherical optics, posing the alignment and stability problems in testing deep concave domes.
Holographic and digital holographic nulls pose significant application problems as well. Traditional film holograms may work in reflection or in transmission. Reflective holograms have to be thick for the mid-IR working wavelength, which makes fabrication difficult. In transmission (combined with a mirror), efficiency of a film hologram would be low, with two passes required. Both types are likely to suffer from stability issues. Digital holograms with dynamic control require liquid crystal (LC) spatial light modulators (SLM). The pixel pitch of such modulators is presently not sufficiently-fine for the large diffraction angles required in dome inspection. The feasible size of the LC SLM is typically smaller than required. Static computer generated holograms (CGH) have the same issues as aspheric null mirrors: difficulty of testing and high sensitivity to misalignment.
Subaperture Stitching Interferometry (SSI) is a popular technique applicable to flat and spherical surfaces. With spherical surfaces, rotation of the optics under test around the center of the sphere provides for capturing multiple interferograms of the subapertures that are later stitched together to provide full aperture wavefront map. A perfect sphere would produce identical wavefront maps from each subaperture, which allows the stitching. Aspheric domes, however, would produce substantially different wavefront maps at different dome orientations. Moreover, the same portion of the dome at a different orientation would produce a different wavefront. For this reason, SSI is not readily applicable to metrology of aspheric domes.
Shack-Hartmann wavefront sensors have inherent limitations in the spatial resolution and in the resolution of wavefront measurement. They usually fall short of the resolution provided by interferometers.
In summary, no tools presently exist for optical metrology on deep concave domes. This invention presents a novel system and process for metrology on aspheric, conformal domes, associated corrector optics, and other highly aberrated optics. The invention is compatible with existing metrology tools and manufacturing processes already used in production of large spherical windows and domes.